1. Field of the Invention:
The present invention relates to a charge-coupled device suitable for use in such apparatuses as an image sensor and a delay element, more particularly, to a charge transfer device having such a structure that no signal charge is left when the signal charge is transferred from vertical transfer channels to a plurality of horizontal transfer channels and a method for driving the same.
2. Description of the Related Art:
As a typical charge transfer device using a charge-coupled device (hereinafter, referred to as a "CCD"), an image sensor is widely known. In recent years, the image sensor tends to be provided with a large number of picture elements so as to increase resolution. In addition, a chip for the image sensor is becoming more and more reduced in its size so as to provide high-density for the image sensor. For these purposes, it is effective to dispose horizontal transfer channels in a plurality of rows for preventing the transfer frequency and the amount of dissipated power from increasing as the number of picture elements increases. It is also effective to dispose the horizontal transfer channels in a plurality of rows so as to reduce the pitch of transfer electrodes in order to provided a CCD with high-density.
Hereinafter, a conventional example will be described, taking a solid-state image pickup device including horizontal transfer channels, which consists of a charge transfer device (hereinafter, referred to as a "CTD") with CCDs in two rows as an example.
FIG. 1 illustrates a plan view showing a boundary area between a plurality of vertical transfer channels 2 and horizontal transfer channels 3 and 4 in two rows in the CTD 1 according to a conventional example. The CTD 1 of the conventional example includes the vertical transfer channels 2 in a plurality of columns. Each of the vertical transfer channels 2 is constituted by a four-phase CCD using a final stage driving signal .PHI.V4. The CTD 1 has a first transfer gate 5 driven by a driving signal .PHI.TG1 between each vertical transfer channel 2 and the first horizontal transfer channel 3, and a second transfer gate 6 driven by a driving signal .PHI.TG2 between the horizontal transfer channels 3 and 4 in two rows.
FIG. 2 shows a cross-sectional view taken along line X9--X9 in FIG. 1. FIG. 3 shows a cross-sectional view taken along line X10--X10 in FIG. 1. In the CTD 1, an N.sup.- layer 8 is formed on a P-type substrate (hereinafter, referred to simply as a "substrate") 7, and then an insulating layer 9 made of, for example, silicon oxide is formed on the N.sup.- layer 8. The following structure is formed on the insulating layer 9 by thin film technology. A P.sup.- region is formed as a barrier layer 19a in the N.sup.- layer 8 between the first transfer gate 5 and the horizontal transfer channel 3. A P.sup.- region is formed as a barrier layer 19b in the N.sup.- layer 8 between an electrode 14 and an electrode 11 of the driving signal .PHI.V4 constituting the transfer gate 5. The configurations similar to that of the barrier layers 19a and 19b are formed as channel stops 20 between the vertical transfer channels 2. The CTD 1 includes electrodes 10 and 11 in a plurality of rows, respectively, extending in a horizontal direction of FIG. 1, i.e., a row direction formed on the insulating layer 9. The electrodes 10 and 11 are formed so that the ends of the electrodes 10 and 11 in a width direction overlap each other. An insulating layer 12 is formed between the electrodes 10 and 11. An electrode 13 is formed so as to overlap the end of the width direction of the electrode 11 on the final stage of the vertical transfer channel 2. The electrode 13 constitutes the first transfer gate 5 together with the electrode 14. The electrodes 13 and 14 extend in the row direction. The other end of the electrode 13 overlaps part of the electrode 14. The electrodes 13 and 14 are insulated from each other by the insulating layer 12. The driving signal .PHI.TG1 is supplied to the electrodes 13 and 14.
A plurality of electrodes 15 and 16 are formed in the vicinity of the end on the side opposite to the electrode 13 of the electrode 14 extending in the row direction. The ends of the plurality of electrodes 15 and 16 of a vertical direction overlap the vicinity of an end on the side opposite to the electrode 13 of the electrode 14. The horizontal transfer channel 3 includes the electrodes 15 and 16 extending in a column direction perpendicular to the row direction. Driving signals .PHI.H1 and .PHI.H2 are supplied to the plurality of electrodes 15 and 16 whose ends in the width direction overlap each other. The electrode 15 is insulated from the electrode 14 by the insulating layer 12, and further from the electrode 16 by an insulating layer 17. The electrodes 15 and 16, as shown in FIG. 1, are bent so as to be inclined with respect to the width direction in the region where the second transfer gate 6 is formed. Passing through the region where the second transfer gate 6 is formed, the electrodes 15 and 16 extend straight in the column direction again.
The transfer gate 6 extends in the row direction and includes an electrode 18 having a length in the column direction, i.e., a gate length L1. The same configurations as those of the channel stops 220 are disposed as channel stops 21 in the transfer gate 6. With this structure, transfer channels 22 extending in a direction indicated with an arrow A1 are formed between the channel stops 21 in the transfer gate 6. The driving signal .PHI.TG2 is supplied to the electrode 18. Each of the plurality of electrodes 15 formed on the electrode 18, as shown in FIG. 3, extends so as to cross the electrode 18 interposing the insulating layer 12 therebetween. On the electrode 18, the plurality of electrodes 16 adjacent to each other are disposed at intervals L2 (FIG. 2) in the direction of the arrow A1, and the plurality of electrodes 15 adjacent to each other are disposed at intervals L3 (FIG. 2) in the direction of the arrow A1.
The substrate 7, on which the above configuration is formed, is covered with a transparent insulating layer 23, and then a light shielding film 24 is formed on the transparent insulating layer 23, as shown in FIGS. 2 and 3. Sequentially, a transparent insulating layer 25 is formed on the light shielding film 24, and then a light shielding layer 26 is formed on the transparent insulating layer 25. The light shielding layers 24 and 26 are made of, for example, aluminum. As shown in FIG. 2, a field insulating layer 30 is formed on the substrate 7 at the ends of the sides opposite to the transfer gate 6 of the plurality of electrodes 15 and 16. A metal wiring 28 made of aluminum and the like is connected through a contact hole 27 formed through the transparent insulating layers 23 and 25 on the field insulating layer 30. The driving signals .PHI.H1 and .PHI.H2 are supplied to the electrodes 15 and 16 via the metal wiring 28.
Voltage potentials in the vertical transfer channels 2, the transfer gate 5, the horizontal transfer channel 3, the transfer gate 6, and the horizontal transfer channel 4 in the conventional CTD 1 are shown in FIG. 4.
In the CTD 1 with the above structure, signal charge horizontally transferred in the second horizontal transfer channel 4 alone is transferred from the first horizontal transfer channel 3 to the second horizontal transfer channel 4, while keeping the signal charge to be horizontally transferred in the first horizontal transfer channel 3 so as not to be horizontally transferred. In order to realize this operation, the signal charge is temporarily stored under the transfer gate 6 provided between the first horizontal transfer channel 3 and the second horizontal transfer channel 4. For this purpose, the transfer gate 6 is required to ensure sufficient storage capacity. Therefore, it is necessary to set the gate length L1 at a large value. As described below, the large gate length L1 causes the insufficient insurance of transfer efficiency when the charge is transferred from the transfer gate 6 to the second horizontal transfer channel 4.
As a result, during the transfer of charge from the transfer gate 6 to the second horizontal channel 4, part of the signal charge is left in the transfer gate 6. The left signal charge flows at random into the first and second horizontal transfer channels 3 and 4 when the signal charge is transferred at high speed in the row direction by the succeeding two-phase first and second horizontal transfer channels 3 and 4. This will cause vertically striped pattern noise on an image screen.
In such a conventional technique, the electrodes 15 and 16 of the horizontal transfer channels 3 and 4 are electrically connected to the metal wiring 28 on the side opposite to the transfer gate 6 with respect to the second horizontal transfer channel 4, as shown in FIG. 2. Moreover, each of the plurality of electrodes 15 and 16 is required to be an independent electrode electrically insulated from each other, to which each of the driving signals .PHI.H1 and .PHI.H2 is independently supplied. In addition, the electrodes 15 and 16 should be electrically separated from each other on the transfer gate 6. As the widths of the electrodes 15 and 16 are reduced and therefore an arrangement pitch is reduced in the horizontal transfer channels 3 and 4 to provide high density for the CCD, it becomes more difficult to satisfy the requirements described above.
If the attempt to meet the above requirements is made by using the conventional technique, the transfer channel 22 should be bent in the transfer gate 6 and the gate length L1 of the electrode 18 of the transfer gate 6 should be set at a large value, as shown in FIG. 1. Even if such a measure is taken, however, the above problems are not perfectly solved.
As a second conventional technique used to solve the problems of the above conventional technique, Japanese Laid-Open Patent Publication No. 4-213282 discloses a device in which the horizontal transfer signal lines are constituted independently from one another in first and second horizontal transfer channels so as not to store charge in transfer gates between a plurality of horizontal transfer channels.
In such a device, however, the metal wiring 28 is electrically connected to the gate electrodes 15 and 16 through a contact portion 29 on the horizontal transfer channels, i.e., an active region. Thus, an alloy reaction, which changes the channel potential, occurs in the contact portion 29. As a result, deterioration of transfer is caused.
In order to avoid the above disadvantages of the second conventional technique, the structure, in which a barrier layer made of polysilicon and the like is provided so as to prevent the metal wiring and the gate electrodes from being directly electrically connected to each other directly above the transfer channel, has been proposed. In this structure, however, the transfer channel is electrically connected to the barrier layer on the gate electrode. Therefore, in terms of design rule, the pitch of the plurality of electrodes of the horizontal transfer channels is required to be 10 .mu.m or more. Thus, such a pitch of electrodes is inadequate to provide a large number of picture elements and high-density for the CCD. Thus, the structure is disadvantageous in that the transfer channel 22 of the transfer gate 6 cannot be provided on prolongations of all the vertical transfer channels 2 in the CTD having the pitch of 10 .mu.m or less along the horizontal direction of the plurality of electrodes 15 and 16 provided for the horizontal transfer channels 3 and 4 shown in FIGS. 1 to 3.